In the art there is an increasing demand for latices having reduced odor. This is relevant for many applications, e.g., the use of latex as latex foam used for pillows or latex sealings used for conserving food.
All of these applications have in common that the latex used has to be low in odor because otherwise the odor of said product is not acceptable to a customer.
However, a latex and articles manufactured therefrom have to fulfill specific mechanical and physical properties. All of these properties are obtained by adjusting the ratio of monomers used in polymerization to the specific properties which should be obtained. Additionally specific additives are present during polymerization which for instance improve the physical properties of the product. In case of latex foams the latex has to provide particular physical properties suitable for processing conditions commonly used. The same applies to sealings made of latex because, for instance in manufacturing cans the equipment used for filling cans cannot be adjusted easily to specific requirements of any latex.
In polymerization commonly tert-dodecyl mercaptan is used as regulator. By adding said regulator the specific mechanical and physical properties of the latex are adjusted. However, mercaptans are sulfur-containing compounds and said mercaptans have a very bad smell since they stink.
Mercaptans used as regulator in polymerization reactions also have a very high boiling point so that it is not possible to remove said regulators from the obtained latex by customary separation methods. Mercaptans used as regulators in the polymerization reaction maintain in the final product.
Besides the bad smell of tert-dodecyl mercaptan the final product is not unhealthy because of said compound.
However, there are applications where the final product has to be free from smell.
In the U.S. Pat. No. 6,818,236 B2 an SBR latex used for manufacturing chewing gum is disclosed. The chewing gum SBR is polymerized by using styrene and 1,3-butadiene as monomers. The copolymerization can be conducted in the presence of n dodecyl mercaptan as chain transfer agent to reduce the molecular weight of the SBR. Moreover, the amount of n dodecyl mercaptan used should be low enough that it is totally depleted by the time the copolymerization is short-stopped.
Short-stopping of the monomer conversion is conducted at about 65% to about 70% conversion rate. Preferably a reducing agent is used for short-stopping the monomer conversion.
The Example of U.S. Pat. No. 6,818,236 is directed to an experimental SBR latex. N-dodecyl mercaptan was charged into the reactor as chain transfer agent. After recovering and drying the SBR latex the obtained latex film had a Mooney viscosity of 80.
According to the teaching of U.S. Pat. No. 6,818,236 the synthesized SBR is low of odor and low in taste because of the fact that it is essentially void of sulfur-containing compounds. This means that according to the teaching of said U.S. patent the amount of sulfur-containing chain transfer agents has to be minimized whereas it is advantageous to short-stop the polymerization reaction at a very low conversion rate. The freedom from odor and taste is not correlated with the use of n-dodecyl mercaptan as chain transfer agent. A person skilled in the art can derive from the teaching of U.S. Pat. No. 6,818,236 only that the amount of sulfur-containing chain transfer agent has to be minimized. U.S. Pat. No. 6,818,236 also refers to a completely different technological background since a latex used for soft chew applications has to have completely different properties compared with a latex used for manufacturing latex foams or latex sealings.
Moreover, a latex used for manufacturing latex foams or latex sealings should preferably have a high solids content and a broad particle size distribution. Furthermore, a solid rubber cannot be processed without using organic solvents, which contribute to VOC and may cause odor.
The latex for the manufacture of latex foam and can sealing may contain a suitable reinforcing latex beneath a base latex known to the persons skilled in the art.
J. Schnetger: “Lexikon der Kautschuktechnik”, 2.edition, 1991 Hüthig Buch Verlag, Heidelberg, only makes a general reference to the suitability of styrene butadiene latices (SBR) for the production of foam rubber. But there is no information derivable with respect to the properties of the latex polymer like Mooney viscosity or which chain transfer agents were used in the preparation of the latex.
GB-A 1546387 discloses the use of a styrene butadiene latex the latex polymer having a Mooney viscosity of 100 to 150 in a sealant composition for food can application. But this prior art reference is totally silent with respect to any odor properties of the latex, nor does it disclose which chain transfer agent was used.
Thus, the technical problem underlying the present invention is the provision of a superior latex useful for manufacturing latex foam, latex sealings or latex films, whereby the obtained latex is low in odor and can be used for manufacturing the aforementioned articles without changing further properties of the latex.
GB-A 1406051 relates to a styrene/butadiene/vinyl pyridine latex, the latex polymer having a Mooney viscosity of 50 to 150, preferably of 17 to 130 used in adhesive compositions for bonding fiber cords to rubber in tires. Vinyl pyridine is the essential component in the latex polymer in order to achieve the desired properties. Although the reference mentions t-dodecylmercaptan and n-dodecylmercaptan as suitable molecular weight regulators in the examples t-dodecylmercaptan is used. Furthermore odor is no problem for the disclosed use of the latex as can be seen from the copolymerization of vinyl pyridine which is a monomer having an unpleasant intensive odor. Thus, GB-A 1406051 can not give any incentive that by using n-alkylmercaptans as chain transfer agent in the preparation of a polymer latex the odor of the resulting latex can be reduced.